In recent years, in a hydraulic driving apparatus for a hydraulic machine equipped with a plurality of hydraulic actuators, such as a hydraulic excavator, a hydraulic crane or the like, a variable displacement type hydraulic pump has included a load-sensing control as disclosed in DE-A1-3422165 (corres. to JP-A-60-11706). The load sensing control controls the discharge rate of the hydraulic pump in such a manner that discharge pressure of the hydraulic pump is raised more than maximum load pressure of the plurality of hydraulic actuators by a predetermined value. In this case, pressure compensating valves are arranged respectively in meter-in circuits for the hydraulic actuators, and the flow rate of hydraulic fluid supplied to the hydraulic actuators is controlled by flow control valves equipped respectively with the pressure compensating valves. By doing so, the discharge rate of the hydraulic pump increases and decreases depending upon the requisite flow rates for the hydraulic actuators, so that economical running is made possible. In addition, by the pressure compensating valves, in sole operation, precise flow control is made possible without being influenced by load pressure of the operated actuator, while, in combined operation, smooth combined operation is made possible without being influenced by the mutual load pressures, in spite of the fact that the hydraulic actuators are connected in parallel relation to each other.
In this hydraulic driving apparatus, there is the following problem peculiar to the load sensing control.
The discharge rate of the hydraulic pump is determined by the displacement volume or, in the case of a swash plate type, by the product of an amount of inclination and rotational speed of the swash plate such that the discharge rate increases in proportion to an increase in the amount of the inclination. In this amount of inclination of the swash plate, there is a maximum amount of inclination as a limit value which is determined from the constructional point of view. The discharge rate of the hydraulic pump is maximized at the maximum amount of inclination. Further, driving of the hydraulic pump is effected by a prime mover. When input torque to the hydraulic pump exceeds output torque from the prime mover, rotational speed of the prime mover starts to decrease and, in the worst case, the prime mover reaches stall. In order to avoid this, input-torque limiting control is carried out in which a maximum value of the amount of inclination of the swash plate is so limited that the input torque to the hydraulic pump does not exceed the output torque from the prime mover, to control the discharge rate.
As described above, there is the maximum-limit discharge flow rate in the hydraulic pump. Accordingly, at the combined operation of the plurality of hydraulic actuators, when the sum of the requisite flow rates for the plurality of hydraulic actuators commanded by their respective operating levers is brought to a value higher than the maximum-limit discharge flow rate of the hydraulic pump, it is made impossible to increase the discharge rate of the hydraulic pump to the requisite flow rate by the load sensing control, so that an insufficient state of the discharge rate with respect to the requisite flow rate occurs. In the present specification, the hydraulic pump is thus said to be saturated when the hydraulic pump is saturated in this manner, a major part of the flow rate discharged from the hydraulic pump flows to the hydraulic actuator on the low pressure side, but the hydraulic fluid is not supplied to the hydraulic actuator on the high pressure side, so that smooth combined operation is made impossible.
In order to solve this problem, in the hydraulic driving apparatus disclosed in the above-mentioned DE-A1-3422165 (corres. to JP-A-60-11706), the arrangement is such that two pressure receiving sections acting respectively in the valve opening and closing directions are additionally provided to each of the pressure compensating valves, arranged in the meter-in circuits for the respective hydraulic actuators. The pump discharge pressure is introduced to the pressure receiving section acting in the valve opening direction, and the maximum load pressure of the plurality of actuators is introduced to the pressure receiving section acting in the valve closing direction. With this arrangement, when the sum of the respective requisite flow rates for the plurality of hydraulic actuators commanded by their respective operating levers is brought to a value higher than the maximum-limit discharge flow rate of the hydraulic pump, the pressure compensating valve for the actuator on the low pressure side is restricted in response to a drop of the differential pressure between the discharge pressure of the hydraulic pump and the maximum load pressure. Thus, the flow rate flowing through the actuator on the low pressure side is restricted and, therefore, it is ensured that the hydraulic fluid is supplied also to the hydraulic actuator on the high pressure side. As a result, the discharge flow rate of the hydraulic pump is divided to the plurality of actuators, so that the combined operation is made possible.
Furthermore, DE-A1-2906670 discloses a hydraulic driving apparatus in which pressure compensating valves different in operation principle from the general pressure compensating valves described above are incorporated respectively in a meter-in circuit and a meter-out circuit for flow control valves. The function of the pressure compensating valve incorporated in the meter-in circuit is substantially the same as that disclosed in DE-A1-3422165. That is, the pressure compensating valve usually makes possible smooth combined operation and flow-rate control not influenced by load pressure. On the other hand, when the hydraulic pump is saturated, the pressure compensating valve senses the saturation, to restrict the pressure compensating valve in the meter-in circuit for the actuator on the low pressure side, thereby making it possible also to supply the hydraulic fluid to the actuator on the high pressure side. Moreover, the pressure compensating valve incorporated in the meter-out circuit functions in the following manner.
When a hydraulic cylinder is driven by hydraulic fluid supplied from the meter-in circuit, the driving speed of the hydraulic cylinder is controlled by flow-rate control in the meter-in circuit. In contradistinction thereto, when a negative load such as an inertial load or the like acts upon the hydraulic cylinder, the hydraulic actuator is forcedly driven so that the pressure of the return fluid from the hydraulic cylinder tends to increase. In this case, for the arrangement provided with no pressure compensating valve in the meter-out circuit, disclosed in DE-A1-3422165 or the like, it is impossible to pressure-compensation-control the flow rate passing through the flow control valve in the meter-out circuit so that the flow rate of the return fluid increases. As a result, a balance in ration is lost between the flow rate of the hydraulic fluid supplied to the hydraulic cylinder and the flow rate of the return fluid discharged from the hydraulic cylinder, so that cavitation occurs in the meter-in circuit. In DE-A1-2906670, the pressure compensating valve is incorporated also in the meter-out circuit, whereby, when the negative load acts upon the hydraulic cylinder, the flow rate passing through the flow control valve is pressure-compensation-controlled with respect to pressure fluctuation in the meter-out circuit, thereby preventing an increase in the flow rate of the return fluid discharged from the hydraulic cylinder to prevent occurrence of cavitation in the meter-in circuit.
In DE-A1-2906670, however, the pressure compensating valve incorporated in the meter-out circuit is not so arranged as to sense saturation of the hydraulic pump. Therefore, there arises the following problem.
When the hydraulic pump is saturated, that is, when the discharge flow rate of the hydraulic pump reaches a maximum-limit flow rate so that the discharge flow rate falls into an insufficient state, the pressure compensating valve for the actuator on the low pressure side is restricted in the meter-in circuit as described previously, to divide the discharge flow rate of the hydraulic pump to the plurality of hydraulic actuators. At this time, however, it is needless to say that the flow rate supplied to each actuator is decreased more than that prior to the saturation. Under the circumstances, if negative load acts upon the hydraulic actuators, the pressure compensating valve in the meter-out circuit attempts to pressure-compensation-control the flow rate passing through the flow control valve in a manner like that prior to the saturation. For this reason, the flow rate of the return fluid from the hydraulic actuators attempts to be brought to a flow rate identical with that prior to the saturation. Thus, the balance in ratio is lost between the hydraulic fluid supplied to the hydraulic cylinder and the flow rate of the return fluid discharged from the hydraulic cylinder, so that cavitation occurs in the meter-in circuit.
It is an object of the invention to provide a hydraulic driving apparatus capable of preventing occurrence of cavitation in either case prior to saturation of a hydraulic pump and during saturation thereof, so that stable operation can be effected.